The present invention relates to a holographic reconstruction system for the three-dimensional reconstruction of object light points of a scene, and to an according method. The holographic reconstruction system comprises spatial light modulator means which modulate light waves which are capable of generating interference with at least one video hologram, focussing means which focus the modulated light waves such that an observer can watch the reconstructed object light points of the scene from a visibility region which is thus generated by way of focussing, and deflection means which position the visibility region by way of directing the modulated light waves.
In a holographic reconstruction system, sufficiently coherent light is modulated by spatial light modulator means (SLM), e.g. an LCD. A diffractive structure, the hologram or a sequence of holograms, is encoded on the SLM. Object light points are generated through interference of the light which is modulated with holograms in the SLM. The entirety of those object light points form the three-dimensional reconstruction of an object or scene. The light of all object light points propagates in the form of a light wave front, so that one or multiple observers can watch those object light points from an eye position as a three-dimensional scene. For the observer, the light appears not to come from the SLM, but from the three-dimensional object reconstruction, i.e. from multiple depth planes. The observer focuses his eyes on the object reconstruction with its multiple depth planes. The eyes can only see the light which is diffracted by the SLM, but not the light which is transmitted directly. When watching a holographic display, an observer thus ideally has the same impression as if they watched a real object. This means that in contrast to a stereoscopic representation, a holographic reconstruction realises an object substitute, which is why the problems known in conjunction with stereoscopy, such as fatigue of the eyes and headache, do not occur, because there is generally no difference between watching a real scene and a holographically reconstructed scene.
Prior art holographic reconstruction systems, e.g. as described by the applicant in the international patent applications WO2004/044659, WO2006/119920 or WO2006/119760, are based mainly on the following general principle: Spatial light modulator means modulate a wave front with holographic information. The modulated wave front reconstructs a three-dimensional scene in the form of object light points in a reconstruction volume, which is positioned in front of one or both eyes of one or multiple observers. The reconstruction volume stretches from the exit surface of a display screen, through which the modulated wave front leaves the reconstruction system, to a visibility region in the far field. The Fourier transform which is created when focussing the modulated wave front (or any other far-field transform) of a video hologram which is encoded on the spatial light modulator means lies in the visibility region. However, the holograms can also be encoded such that the object light points do not only appear in front of but also on and behind the display screen, so that the reconstruction volume does not only lie in front of the display screen, but continues beyond that screen and that the observer perceives parts of the reconstructed three-dimensional scene in front of, on or behind the display screen.
The visibility region which is generated by way of focussing has the size of an eye or eye pupil, for example. In this case, a second wave front which is directed at the other eye must deliver a second reconstruction which differs in parallax, so that the other eye is provided another visibility region. If each eye of an observer is situated in a visibility region, the observer can watch the holographically reconstructed scene. The wave fronts which are directed at the different eyes are typically spatially or temporally interleaved with prior art autostereoscopic means. Spatial frequency filters prevent optical cross-talking between the wave fronts. If multiple observers are served, a correspondingly multiplied number of visibility ranges is provided e.g. by way of time- or space-division multiplexing.
In order to maintain a certain clarity, the description below relates mainly to the alignment of a single wave front of the holographic system. The reconstruction system can realise further wave fronts in analogy to the first one, if required. It appears to those skilled in the art that the idea of this invention can be applied as often as necessary for this, depending on the actual number of wave fronts. When doing so, functional elements of the invention can preferably be used commonly for multiple wave fronts.
Alternatively, it is also possible to generate a visibility region which covers both eyes of an observer. However, the size of the visibility region depends on the focal length of the holographic reconstruction system, the wavelength of the used light and the pixel pitch (distance between the centres of two adjacent pixels) of the spatial light modulator which is used for encoding the scene to be holographically reconstructed. The larger the desired visibility region the higher must be the resolution of the SLM used. In order to get a large visibility region, the SLM must have very small pixel apertures which cause great diffraction angles, i.e. the SLM must also have a small pixel pitch and, consequently, a large number of pixels.
In order to reduce the necessary resolution of the SLM, the size of the visibility region can for example be decreased to the size of an eye pupil. However, this may lead to problems with the visibility of the three-dimensional reconstruction, if the observer eye is only partly situated inside the visibility region. Already a slight movement of the observer may cause effects such as disappearance of visibility, vignetting or distortion of the spatial frequency spectrum. Moreover, the borders of the reconstruction volume are difficult to find for an observer whose eyes are situated outside the visibility region. It is therefore necessary for the position of the visibility region to be adapted to the new eye position if an observer moves.
Prior art systems comprise an eye finder for detecting an eye position, and deflection means, for example a mirror, for directing the visibility region at the eye position. The required angular position of the deflection means is found based on the detected eye position, and the deflection means are controlled accordingly in order to match the position of the visibility region to the eye position. When the deflection means have matched the visibility region to the eye position, the hologram for the addressed position is encoded and the three-dimensional scene is reconstructed. Then, the next eye position is detected and so on. This causes the deflection means to move intermittently, which is difficult to be realised using conventional means, in particular at high frequencies, e.g. higher than 20 Hz, as they are easily required when realising a colour multiplexing mode or when simultaneously serving multiple observers.
With a small visibility region, it is further required that the eye finder detects the eye position with a very high accuracy. For example, if the size of the visibility region is between 5 to 10 mm, the eye finder should detect the eye position with a maximum error of about 1 mm. Again, this is difficult to be realised using conventional means.